Molding method

ABSTRACT

The present invention relates to a molding process using a silicate binder substantially based upon an alkali silicate having a higher molecular ratio of silica to alkali. More particularly, it relates to improvements in a molding process, wherein the mold and core are cured by gassing with exceedingly diluted CO 2  and the CO 2  gas consumption for the curing of the molds is reduced to between about 1/2 - 1/20 compared with that of the conventional CO 2  curing process. The resulting molds and cores exhibit an excellent collapsibility after casting, develop no harmful gas in curing and casting procedures, and the used waste sands do not cause any serious soil or water pollution problems.

DEFINITIONS

The term "alkali silicate" denotes a water-soluble silicate consistingessentially of silica and monovalent alkali ions including sodium,potassium, lithium, and ammonium ions. The term "silicate binder"denotes a binder substantially based upon alkali silicates. The term"molecular ratio of silica to alkali" denotes the molar ratio of SiO₂/M₂ O, wherein M denotes a monovalent alkali ion, and this term isabbreviated as "M.R.".

BACKGROUND OF THE INVENTION

It has long been known, that the CO₂ molding process requires sodiumsilicate of so-called "lower molecular ratio" as a binder, and the moldis cured by gassing with either pure or concentrated CO₂, the "lowermolecular ratio" is understood to denote a ratio of between about 2-2.5.Since over 25 years it has been believed that silicates having a highermolecular ratio are not suitable as a CO₂ process binder because theycan not provide sufficient bonding strength to the mold. Thus thesilicate having M.R. 2.7 has rarely been used as a CO₂ process binder.

Although the conventional CO₂ cured mold which is bonded with lower M.R.silicates shows a high bonding strength, it has several essentialdisadvantages: -- Firstly, it has a very poor collapsibility aftercasting. Secondly, it requires a large consumption of curing CO₂, e.g.,as much as the same amount by weight as that of the binder by weight.Consequently, the cost of CO₂ for the molds amounts to about 2-2.5 timesthat of the binder. Furthermore, the used sands from the conventionalCO₂ process molds can not be reused beneficially and most of the usedsands have to be discarded as waste, which inevitably causes undesirablesoil and water pollution problems due to the higher alkali content andthe higher water solubility of the lower M.R. silicate binder in theused sands. Since the CO₂ process had come into wide use, thesedisadvantages caused the CO₂ process to be replaced by other moldingprocesses using, e.g., organic binders.

Much effort has since been expended to overcome the poor collapsibilityof CO₂ process molds by admixing different kinds of break-down agents.Nevertheless there is much to be desired in admixing break-down agentsbecause a large amount of such additives usually provides a poor surfacestability and a lower strength to the molds. Furthermore, organic orcarbonaceous break-down agents do not work at temperatures higher than1000° C., to which the molds for iron and steel casting are subjected.It has been proposed to dilute the CO₂ with air in the conventional CO₂process in order to reduce the CO₂ consumption, but it has been provedthat the CO₂ consumption can be reduced no more than 30% by this methodcompared with pure CO₂ curing, when the lower M.R. silicate binder isemployed as described in Table 4 of the detailed description, this issupported by the following report: K. Hara et al.: "Application of CO₂-Air mixing Gas to CO₂ Process" The Journal of the Japan Foundrymen'sSociety, No. 4, Vol. 39, p. 64, 1967.

On the other hand, it has been suggested that a better collapsibilitywould be achieved by using a sodium silicate binder having a relativehigh M.R. of about 3. But there were no effective curing methodsavailable in practice expect for the following two methods: -- Firstly,heating the mold up to 250°-300° C. for a period of more than half anhour, which leads to a very low productivity and is uneconomical.Secondly, curing with dried or hot air, which can not uniformly cure thewhole body of a large mold in practice because the moisture-saturatedair will form water-saturated layers next to the dried layer.Furthermore, it takes at least 10 minutes to cure the test mold piececompletely and the injection of dried air blows off the sand particlesaround the blowing hole. Thus the dried air injection can not beemployed in practice.

According to the conventional CO₂ process, such a mold which was bondedwith higher M.R. silicate binder could not be cured properly in theusual working process on account of the following reasons: -- A moldbonded with sodium silicate having a M.R. of e.g. 3.16, can be curedrapidly by the conventional gassing method, but the bonding strengthwill so rapidly be lost in the short lapse of time after curing, thatone day after gassing the mold will turn practically useless forcasting, the silicates having a much higher M.R. than 3.16, as shown inTable 1 below are even more useless in comparison with the presentinvention. Thus the sodium silicate having a M.R. of not less than 2.7could not be used beneficially as a molding binder for the conventionalCO₂ process. The other alkali silicates could not be used due to thesame phenomena.

Contrary to the above described prior art, it has now been found whysuch a rapid deterioration of the bonding strength of a once cured moldwhich is bonded with a higher M.R. silicate occurs. The rapiddeterioration of the bonding force is based essentially on thehypersensitivity of the higher M.R. silicates toward CO₂. It has furtherbeen found based on the above, that the mechanism of developing a strongbonding strength with a higher M.R. silicate does not depend on theshocking and complete gelation, especially not on being shocked togelation by concentrated CO₂ injection. If the silicate film coating ofthe sand particles is shocked to gelation by gassing with CO₂ accordingto the prior art, the silicate film loses it's bonding strength quiterapidly after being cured. The lower M.R. silicate binder is not sosensitive as the higher M.R. silicate, therefore the concentrated CO₂injection can provide a strong bonding force except when too much CO₂ isinjected, e.g., the mold is overgassed.

Further according to the above there is provided a proper method toovercome the aforementioned problem in the prior art, i.e., wherein adilution agent for the CO₂ is introduced to the gas in such an amountthat the agent may inhibit the reaction between the higher M.R. silicatebinder and CO₂. By diluting the CO₂ exceedingly with the inhibitingagent to a CO₂ concentration of not more than 20%, the mold sands whichare admixed with the higher M.R. silicate can be cured properly.Furthermore, the CO₂ consumption for curing the mold is reduced to about1/20-1/2 compared with that in the prior art process.

Furthermore, many other disadvantages due to the lower M.R. silicatebinder, e.g., soil or water pollution problems caused from the highalkalinity of the used waste sands and difficulties in the recovery ofthe sand can be either eliminated or reduced by the application of ahigher M.R. silicate binder.

OBJECTS OF THE PRESENT INVENTION

Accordingly, the first object of the present invention is to provide amolding process wherein the curing CO₂ consumption is much less thanthat in the prior art processes.

The second object of the present invention is to provide a moldingprocess wherein the resulting molds exhibit excellent collapsibility.

The third object of the present invention is to provide a moldingprocess wherein the used sands do not cause serious soil and waterpollution problems.

The fourth object of the present invention is to provide a molddeveloping no harmful gases in curing and casting procedures.

The fifth object of the present invention is to provide a mold the usedsands of which can be re-used beneficially.

The seventh object of the present invention is to provide a moldingprocess wherein the mold is cured in a very short time and can be drawnoff from the pattern immediately after curing.

In general, the object of the present invention is to provide a moldingprocess and a mold which is excellent from all points of view, i.e.,having minimum CO₂ consumption, having an excellent collapsibility,causing no serious soil-, water- and atmospheric pollution problems, andproviding a possibility of reusing the used sands as well as rapidcuring.

SUMMARY OF THE INVENTION

The present invention relates more particularly to improvements in amolding process wherein a sand mixture admixed with a silicate binderhaving higher molecular ratio of silica to alkali is properly cured bygassing with exceedingly diluted CO₂ in a short period of time.

The alkali silicate binder having a higher molecular ratio whichessentially based upon sodium, potassium, lithium and/or ammoniumsilicate is mixed with sands, then the sand mixture is rammed into themold and gassed with exceedingly diluted CO₂ which diluted with a gaswhich is inert towards alkali silicates to a CO₂ concentration of notmore than 20 v/v %. The CO₂ consumption for curing in this process isreduced to an amount of between 1/2-1/20 compared with that in theconventional CO₂ process. While the resulting mold has enough green andhot strength for iron and steel casting, the mold exhibits excellentcollapsibility after casting. The lower alkali content in the highermolecular ratio silicate binder provides good re-use characteristics tothe sands. The silicate binder contains no detrimental substance. Thusprovides the advantages that the resulting molds develop no harmful gasin both curing and casting procedures and the waste sands cause noserious soil and water pollution problems.

In particular according to the present invention there is provided amolding process which comprises the steps of (a) forming a moldingcomposition by admixing at least one alkali silicate binder selectedfrom the group consisting of a sodium silicate having a molecular ratioSiO₂ alkali metal oxide of from 2.7 to 4.5, a potassium silicate havinga molecular ratio SiO₂ alkali metal oxide of from 2.5 to 5.0, a lithiumsilicate having a molecular ratio SiO₂ alkali metal oxide from 2.0 to5.5 and an alkali metal-ammonium silicate having a molecular ratio SiO₂alkali metal oxide from 2 to 9 to a refractory material (b) ramming themolding compositions into a mold and (c) then gassing the moldingcomposition with a dilute CO₂ gas essentially consisting of CO₂ and agas which is inert towards alkali silicate wherein the CO₂ --concentration is not more than 20 v/v%. Suitably, the alkali silicatebinder has a molecular ratio of 2.7-4.5 in particular of 3.0-4.5.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a cross-sectional view of a collapsibility testing castpiece (DIA): diameter at the bottom is 250 mmφ. (1): core, (HGT): heightof cast piece 100 mm.

FIG. 2 shows the bottom view of the testing piece shown in FIG. 1. (1):core.

FIG. 3 shows a flow sheet of a collapsibility testing method of coresafter casting. (1): core, (WT): dropping bar weight 2kg, (HGT); heightof drop 200 mm, (DIA): diameter of bar weight 25 mmφ, (α): angle ofconetop 45°.

FIG. 4, FIG. 5 and FIG. 6 show collapsibility diagrams of tested coresincluding comparative samples which are prepared by a conventional CO₂process.

Abscissa: dropping times = number of droppings of the dropping barweight in log scale

Ordinate: weight of broken-down sands in gram

Energy per 1 drop: 0.4 kg.m. The reference numerals shown at each curvecoincidents with the numeral of the corresponding example of the presentinvention listed in one of the Tables 4, 5, 6 and 8, while the numeralswith the prefix "R" refer to the comparative samples which are preparedby a conventional CO₂ process. In FIG. 5. collapsibility diagrams ofexamples containing higher M.R. alkali silicates are shown.

FIG. 6 shows the effect of higher M.R. alkali silicates as compared withthat of the lower M.R. alkali silicates comparative samples (R9) and(R10). The higher M.R. alkali silicate containing example (25) includingno additives shows a much better collapsibility than (R10) includingsucrose. (21)-(24) contain organic inhibitors. (12) containscarbonateous material.

FIG. 7-FIG. 10 show diagrams of the green strength of the samples atdifferent storage times and of the retrained strength after the sampleshave been subjected to high temperatures.

Abscissa: storage time of the mold after gassing in hours (HR) andheating temperature in centigrades.

Ordinate: compression strength in kg/cm²

FIG. 7 corresponds to FIG. 4 using Seto-Jinya sands.

FIG. 8 corresponds to FIG. 5, (6) and (7): Seto-Jinya sands, example(12) contains carbonaceous material while the other examples do notcontain any additives.

FIG. 9 shows the effect of admixing an inhibitor e.g., ammonium silicate(16a) and colloidal silica (17) compared with not adding any inhibitor(19a) to the mixture of same alkali silicate having a M.R. of 3.47 andFlattery sands. Example (20) contains an alkali silicate having a M.R.of 3.66.

FIG. 10 shows effect of admixing organic inhibitors to higher M.R.alkali silicates (21,28) and to the conventional lower M.R. (R11) and(R9). Full lines show Flattery sands while dotted lines show JapaneseMikawa sands. (21,25,28,29) are based on the same higher M.R. alkalisilicate M.R. 3.47.

DETAILED DESCRIPTION

As described above, there was no good process available for preparing amold bonded with a higher M.R. silicate binder, although the higher M.R.silicate binder has many advantages. The dry air injection or suctionrequires too much time for curing the whole body of the mold, whichmakes this method unpractical. CO₂ gas dilution with air to a CO₂ -concentration in the range of between about 50-70 v/v% of the dilutedgas can not properly be employed for curing the higher M.R. silicatebinder. This CO₂ dilution with air was only proposed to reduce CO₂consumption in the prior art processes using a lower M.R. silicatebinder. The maximum reduction was 30%. It has long been recognized thatfurther dilution of the CO₂ to a CO₂ -concentration of less than 50%will rather prolong the total gassing time and the CO₂ consumption cannot be reduced any more.

In Table 1, there are listed test samples prepared by the prior artprocesses, i.e., curing by 100% CO₂ using alkali silicates havingdifferent M.R. values, e.g., 2.25 (lower): 3.16 and 4.1 (higher).(R1)-(R4c) show that the conventional CO₂ injection can not give astrong bonding force to the higher M.R. silicate binder. Testing piecesare made according to JIS Z-2603 and dimensioned to 50 mmφ × 50 mmheight. "S.S.I" denotes the Surface Stability Index which represents theremaining weight of the tests piece after it has been shaken for 1minute on the 6-meshes-sieve in % by weight of the initial weight. (R6)and (R7a,b) show the conventional CO₂ mold, wherein the CO₂ ratioamounts to more than 220% during a gassing time of 10 seconds. A lowergassing pressure and a shorter gassing time than those which are listedin the tests are not enough and suitable in the practice of largemold-curing.

All test samples which are prepared by the prior art processes areprefixed with "R". Employed sands are listed in Table 2, and employedsilicate binders including those in the comparative samples are listedin Table 3.

The reaction between the alkali silicate and CO₂ proceeds according tothe following formula: M₂ O · nSiO₂ + CO₂ →M₂ CO₃ + nSiO₂ wherein nrepresents the molecular ratio of silica to alkali abbreviated as M.R.,M denotes a monovalent alkali ion and consists essentially of amonovalent alkali metal ion and/or ammonium ion. (The ammonium silicatehas rather different properties and will be described afterwards). Thepresent invention is based essentially upon the recognition of the factthat the character "n", i.e., the M.R. determines the reactivity of thesilicate towards CO₂. As shown by (R1) in Table 1, the bonding force ofthe higher M.R. silicate is completely lost one day after gassing, whichfact has proved that the shocking gelation by concentrated CO₂ providesno bonding force to the higher M.R. silicate binder. Subsequentexperiments testing the reactivity of higher M.R. silicate towards CO₂on exposure have disclosed the following features. While the reactivityof the sodium silicate varies very slowly between silicates having aM.R. of about 2-2.5, it increases with a progressively increasing degreewhen the M.R. is increased by more than about 2.7-3.0, and finally, itreaches an ultimate hypersensitive state. For example, a sodium silicatehaving a M.R. of 4.5 exhibits very labile features.

Based on the above it has been further recognized that: The chemicalbonding of a higher M.R. silicate binder cured with CO₂ provides aproper bonding force only when the silicate film is gelled properlyunder mild conditions, i.e., is reacted with CO₂ only in a mildprocedure. In the case that the silicate will be shocked to gelation,the binder will lose it's bonding force quite rapidly after gassing.Such a phenomenon is obviously not observed in the conventional CO₂process because the reactivity of the lower M.R. silicate is very dullin comparison with that of the higher M.R. silicate. The higher M.R.silicate is directly subjected to such a shocking gelation due to it'shigh reactivity to CO₂ although CO₂ is a weak acid for itself.

In order to avoid such a shocking gelation, there is provided a processfor inhibiting the reaction between the silicate and CO₂. Such purposemay be achieved by two methods. Firstly, the CO₂ gas is mixed with agaseous inhibitor, i.e., is diluted with a gas which is inert towardsalkali silicate. Secondly, the silicate binder may be admixed with anaqueous or solid reaction inhibitor.

The first method consists of diluting the CO₂ with a gas which is inerttowards alkali silicate to a CO₂ -concentration of not more than 20 v/v%of the diluted gas mixture. The gaseous diluting agent consistsessentially of air, oxygen, nitrogen, hydrogen, and/or an inert gas suchas helium, argon etc., preferably compressed air.

The compressed air may be dried or heated, which however is notindispensable, but heating the air to temperatures higher than 100° C.is not desirable because of the reasons which are mentioned above in the"BACKGROUND". For example in an embodiment of the present invention, thecompressed air having an atmospheric relative humidity of about 50-95%is employed.

The concentration of CO₂ ranges from more than atmospheric CO₂concentration to not more than 20%. A proper concentration range of CO₂depends upon the M.R. of the silicate and the type of the alkalisilicate. Potassium silicate is more sensitive than sodium silicate, andthe lithium silicate having the same M.R. is even more sensitive.Therefore the employable lowest M.R. in the present invention forpotassium and lithium silicate is lower than that of sodium silicate,i.e., about 2.5 and 2.0 for potassium and lithium silicate respectively.The maximum limit of the M.R. of potassium and lithium silicate is about5.0 and 5.5, respectively.

Besides the CO₂ dilution which is an essential method to inhibit saidreaction, there are additional auxiliary ways consisting of admixing aninhibiting agent to the silicate binder or sand mixture. Among numerousmaterials, some inorganic or organic materials are suitable for thispurpose, i.e.: Colloidal silica is inert towards CO₂ andalkali-stabilized colloidal silica may be admixed with the alkalisilicate without causing gelation provided the mixing is carried outwith care since the mixture remains stable for only a short period oftime. The admixed colloidal silica exhibits an inhibiting effect on thereaction with CO₂ and furthermore, plays the role of a break-down agentdue to the silica content, which influences the break-down properties insuch a way as if the M.R. of the used silicate had been furtherincreased. The term "alkali-stabilized" is understood to mean"stabilized in an alkaline pH range". The ratio of the alkali silicateto said colloidal silica is 100:0.1-1:2, preferably 100:1-5:1 on thebase of silica.

Ammonium silicate, e.g., quaternary ammonium silicate, exhibits a goodinhibiting effect and can be admixed to the alkali silicate in anyproportion to give a stable mixture. Ammonium silicate is fairlyinactive in comparison with alkali metal silicate having higher M.R. andreacts very slowly or slightly with CO₂ at the concentration which itemployed for curing said higher M.R. alkali silicate binder.Accordingly, it exhibits good inhibiting properties. A good break-downis also achieved at relatively low temperatures of between 200°-400° C.as well as at higher temperatures. Furthermore the strength of the moldin the heat is enhanced by admixing ammonium silicate. This is veryimportant for the mold. The ratio of the alkali metal silicate to saidammonium silicate is 100:0.1-1:10, preferably 100:1-1:3 on the base ofsilica.

An alkali metal-ammonium silicate exhibits equally good properties asthe alkali metal silicate admixed with ammonium silicate. Quaternaryammonium silicate is a composition consisting of silica and a quaternaryammonium base represented by the following formula: ##STR1## whereineach of R₁, R₂ and R₃ denotes alkyl or hydroxyalkyl having 1-20 carbonatoms and R₄ denotes alkyl having 1-4 carbon atoms. The alkalimetal-ammonium silicate comprises the above quaternary ammonium base.The ratio of silica to alkalimetal oxide in the alkali metal-ammoniumsilicate is 2:1-9:1 on the base of silica. The above colloidal silicaand ammonium silicate can be both admixed with the silicate binder."Alkali metal" comprises sodium, potassium and lithium. Examples usingcolloidal silica or ammonium silicate are listed in Table 7. Themolecular ratio of the ammonium silicate (SiO₂ /M₂ O, that is SiO₂/ammonium base anhydride) suitably is from about 1/2-7.

Besides the materials based essentially on silica, some organicmaterials can inhibit said reaction and also function as a break-downagent. Such materials are saccharides, polysaccharides, and polyhydricalcohols. Saccharides comprise glucose, fructose, mannose and invertsugar which is a mixture of monosaccharides. Polysaccharide means a di-,tri- or other polysaccharide including sucrose, dexistrin, starch andsoluble starch. Polyhydric alcohol means an alcohol having not less than2 hydroxyl groups and includes mannitol, sorbitol, glycerin and glycol.Sucrose, sorbitol and mannitol are employed in examples of the presentinvention. The above materials are essentially water-soluble andnon-reactive against silicate and CO₂. Furthermore, they do notseriously deteriorate the bonding force of the silicate binder. Whilemost of the above organic materials have long been known as break-downagents in the conventional CO₂ process, there is much left to be desiredin the break-down property, provided the lower M.R. silicate isemployed. In combination with a saccharide, a polysaccharide or apolyhydric alcohol, suitably an alkali silicate having a molecular ratioof 3-4.5, particularly a sodium silicate having a molecular ratio of3.3-4.5 is used.

In the present invention, the above organic materials exhibit firstlyinhibitor, and secondly break-down agent properties. In the case thatthe used sands are recovered, it is preferable to employ the aboveorganic materials because these materials provide a longer moldable lifeto the sand mixture by inhibiting reactions of the binder with manyreactive substances in the used sands. The above organic materials canbe admixed in amounts of 0.1-30 w/w% preferably 1-20 w/w% of thesilicate binder. Examples using organic inhibitors are listed in Table8.

As for soil or water pollution problems, it is preferable to employ asilicate having M.R. higher than 3.4. When the used sands according tothe present invention are dispersed in water, the dispersion shows a pHvalue less than 9, 20 days after being once neutralized by aluminumsulfate, while dispersions of the used sands bonded with the lower M.R.silicate binder show a pH value of 10-12 after several neutralizations.The total amount of dissolved Na₂ O in the cold water from sandsaccording to the present invention amounts 27 w/w% of the total amountof Na₂ O in the used sands.

The recovery of the sand can be carried out efficiently in the presentinvention, because the binder includes less alkaline substance.Preferably, the above-mentioned inhibitors are employed so that thetotal alkali content in the sands may be reduced. The used sands containmore alkali than the new but the collapsibility will be damaged only alittle because the collapsibility depends on the M.R. of the silicate.

The present invention can be applied to all kinds of molds and cores,e.g., having a weight of from 10g-10t for casting of nonferrous orferrous material, preferably to molds and cores which require a quickcuring. The initial compression strength of just cured molds ranges from1 to 4 Kg/cm², which strength is enough to draw off the cured mold fromthe pattern. The compression strength reaches its maximum in one dayafter gassing.

EXAMPLES

The following examples are listed as preferred embodiments of thepresent invention but do not consititute a limitation thereof. Theseexamples are listed in Tables 4-8 and shown in FIGS. 1-10. Table 2-3show the sands and binders employed in the tests. The comparativesamples which are prepared by the prior art process are marked in TableI and in the above Tables and Figures by the prefix "R". Testing moldsand cores are prepared as described above and cured by gassing withdilute CO₂. The CO₂ concentration, gassing time, CO₂ consumption in CO₂ratio, compression strength and retained compression strength, andSurface Stability Index are measured and described. The collapsibilityis tested as follows: The testing cast piece (FC 30,32kg) shown in FIG.1 and FIG. 2 is casted at 1,400° C. The collapsibility of the core isdetermined as the weight of broken-down sands as is shown in FIG. 3, bydropping the bar-weight from the height of 200 mm above each sand level.The resulting data are shown in FIG. 4- FIG. 6.

FIGS. 7-10 show the green strength at different storage times (or benchtime) after gassing and the retained strength of the molds includingcomparative samples. (However, it has been proved that the retainedstrength at temperatures higher than 1000° C. does not represent thecollapsibility in the conventional CO₂ molds.)

Table 4 shows fundamental examples of the present invention andcomparative samples. FIG. 4 corresponds to Table 4.

Table 5 shows the various upper or lower values of the CO₂ concentrationand the M.R. of sodium silicate in the present invention. Here is shownthe CO₂ consumption of large molds as CO₂ ratio, wherein CO₂ ratio forthe molds of weight 2Kg is representative of much larger molds. Comparedwith (R8) according to the prior art the CO₂ consumption according tothe present invention is reduced to about 1/2 (example 14)-1/20 (example11 or 13).

Table 6 shows comparative samples using Mikawa (R9, 10) and Flattery(R11) sands, wherein the latter exhibits a quite high retained strength.(R10) is a comparative sample containing sucrose which is prepared bythe conventional CO₂ process and which demonstrates a relative lowstrength (11.1 Kg/cm²) and a lower collapsibility than the examples ofthe present invention including no break-down agent. The above can beseen more clearly in FIG. 6.

Table 7 shows a lithium silicate binder containing example (15),examples containing ammonium silicate (16a,16b) and colloidal silica(17,18) using Flattery sands. In examples (16a,16b) a quaternaryammonium silicate (methyl triethanol ammonium silicate, SiO₂ 30%) isemployed. As is well known, the green and retained strength depend uponthe sands (silica purity, particle form, grain size distribution etc.).While Seto-Jinya sands (Table 4, FIG. 7) provide a lower strength,Mikawa sands provide a higher strength and Flattery sands an even higherstrength. FIG. 7 and FIG. 8 are shown to provide a better understandingof the listed data in Tables 1-8. The mold-heating under argon flowsimulates the heat hysteresis of a usual mold for ferrous casting. Thedata corresponds relatively well to the collapsibility data shown inFIGS. 4-6.

Table 8 shows the examples containing organic inhibitors. In FIG. 6,(21-24) correspond to the above equally numbered examples. (25) includesno additive. All examples listed in Table 8 exhibit excellentcollapsibility in comparison with the comparative examples (R6-R10).

Example (12) in FIG. 5 and in Tables 5, 6 shows the effect of admixingcarbonaceous material as a break-down agent to the higher M.R. silicate.A wide variety of known additives may be employed in the scope of thepresent invention if a much better collapsibility is desired.

In general, the present invention provides a new molding process,wherein molds are bonded with a higher M.R. silicate binder by curingwith exceedingly diluted CO₂ and at highly reduced CO₂ consumption.Furthermore, the resulting molds exhibit excellent collapsibility,develop no harmful gas and contain no detrimental substance in the usedsands, and the used sands can be efficiently re-used, due to their lowalkalinity. In the scope of the present invention, a wide variety ofembodiments may be done without departing from the gist of the presentinvention. The dilute CO₂ used according to the invention may be heatedto a temperature of not more than about 100° C.

                                      TABLE 1 (1)                                 __________________________________________________________________________                  Curing*           Compression                                   Binder        Co.sub.2                                                                             gassing                                                                             CO.sub.2                                                                            strength kg/cm.sup.2                                                                         S.S.I.***                     Reference (2) pressure**                                                                           time  ratio                                                                              stored time Hrs w/w                           No.   M.R.                                                                              w/w%                                                                              kg/cm.sup.2                                                                          sec.  w/w %                                                                              1    2     24   %  Remarks                    __________________________________________________________________________    R1    4.1 5.5 0.8    7     168  6.1 6.1 0        0                            R2a   3.16                                                                              5.5 0.8    15    366  5.5 7.1 3.6     80.2                          R2b   3.16                                                                              5.5 0.8    30    712  6.45                                                                              --  2.6     78.3                          R3a   3.16                                                                              5.5 0.8    15    338  6.3 3.8 1.3     62.1                                                                             rammed at 30 min.          R3b   3.16                                                                              5.5 0.8    30    726  4.9 --  0.7     14.4                                                                             after mulling              R4a   3.16                                                                              6.0 0.55   2.5    40  3.15                                                                              --  2.35    90                            R4b   3.16                                                                              6.0 0.55   5      93  3.4 --  3.6     91   Potassium silicate       R4c   3.16                                                                              6.0 0.55   10    134  4.3 --  4.3     88.4                          R6    2.25                                                                              6.0 0.8    10    221  2.95                                                                              --  34.5    99.8                          R7    2.25                                                                               5.09                                                                             0.8    10    236  6.4 --  11.8    99.4                          R7b   2.25                                                                               5.09      0.8   15   385 6.8 --      9.7                                                                              99.5                       __________________________________________________________________________     *CO.sub.2 concentration :100%                                                 **gauge pressure                                                              ***S.S.I. : Surface Stability Index                                           (1) Seto-Jinya sands (Japanese)                                               (2) % by weight on the base of sands                                     

                  Table 2                                                         ______________________________________                                        Chemical                                                                      in-        Grain size distribution of sands                                   gredient   w/w % on the sieve                                                 SiO.sub.2  Meshes                                                             Sands W/W %    28     42   60   80   100  200  <200                           ______________________________________                                        S     98.1     2.7    54.0 26.3 10.6 3.0  3.2  0.2                            MK    97.2     12.3   24.0 24.9 30.4 5.0  2.9  0.5                            F     99.8     0.2    14.9 36.2 37.6 8.6  2.2  0                              ______________________________________                                         NOTES;                                                                        S: Seto-Jinya sands, Japanese                                                 MK: Mikawa sands, Japanese                                                    F: Flattery sands, Australian                                            

                  Table 3                                                         ______________________________________                                             Re-                                                                      Ex.  ference         Concentration                                                                           Na.sub.2 O                                                                          K.sub.2 O                                                                           SiO.sub.2                          No.  No.      M.R.   Be degrees*                                                                             w/w%  w/w%  w/w%                               ______________________________________                                        1    R1       4.1    30        5.51  --    21.89                              2    --       3.75   32.6      6.32  --    22.99                              3    --       3.47   36        7.39  --    24.87                              4, 5 R2, R3   3.16   42        9.33  --    28.46                              --   R5       2.25   35        9.20  --    20.10                              --   R6, R7   2.25   50        13.80 --    30.10                              6    R4       3.16   30        --    9.50  19.12                                7**                                                                              --       3.75   32.6      4.41  2.76  23.16                              ______________________________________                                         NOTES;                                                                        *Baume degrees at 20° C                                                **Na-K-Silicate                                                          

                                      TABLE 4.sup.(1)                             __________________________________________________________________________                              Compression                                                                           Surface                                                Curing     CO.sub.2                                                                          strength kg/cm.sup.2                                                                  stability                                   Example                                                                            Binder     Gassing                                                                             ratio                                                                             stored time                                                                           index %                                     No.  M.R.                                                                             %* CO.sub.2 %**                                                                       time sec                                                                            %***                                                                              1   24 hrs                                                                            (S.S.I.)                                                                           Remarks                                __________________________________________________________________________    1    4.1                                                                              7.14                                                                             2.68 12    7.5 1.3 6   98.9                                        1b   "  "  "    18    11.5                                                                              1.8 5.5 98.2                                        2    3.75                                                                             6.33                                                                             "    12    8.2 1.5 9.6 98.7 added water 0.2% on the base           2b   "  "  "    30    20.5                                                                              2.8 9.1 97.6 of sands                               3    3.47                                                                             5.8                                                                              "    15    9.8 1.2 11.5                                                                              99.3 water 0.5%                             3b   "  "  "    30    21.0                                                                              2.2 9.8 99.1   "                                    4    3.16                                                                             6.43                                                                             5.0  10    12.1                                                                              0.6 11  98.4                                        4b   "  "  "    15    18.3                                                                              1.2 11.1                                                                              98.7                                        5    "  5.36                                                                             10.2 7.5   23  2   11.5                                                                              98.3 water 0.54%                            5b   "  "  "    15    45  2.7 8.0 98.5   "                                    6    "  6  5.24 15    21.4                                                                              2.2 11.4                                                                              99.4 Potassium silicate                     6b   "  "  "    30    42.4                                                                              4.5 9.7 99.6   "                                    7    3.75                                                                             6  2.68 12    9.0 2.75                                                                              6.9 99   Na-K-silicate                          7b   "  "  "    18    13  2.14                                                                              5.4 97.9   "                                    R5a  2.25                                                                             5.5                                                                              16.7 15    68.1                                                                              0.3 4.7 98                                          R5b  "  "  "    22.5  103.3                                                                             0.9 4.4 97.3                                        __________________________________________________________________________     *: % by weight on the base of sands                                           **: % by volume and the base of dilute gas                                    ***: CO.sub.2 % by weight on the base of sands of mold                        .sup.(1) Seto-Jinya sands                                                

                                      TABLE 5*                                    __________________________________________________________________________                                      Compres-                                                                      sion                                        Binder         Curing method      strength  Mold                                  M.R.       Gassing   temp-                                                                             CO.sub.2                                                                           kg/cm.sup.2                                                                             (core)                            Ex. SiO.sub.2                                                                            CO.sub.2                                                                          time pressure                                                                           erature                                                                           ratio                                                                              stored time                                                                         S.S.I.                                                                            Weight                            No. M.sub.2 O                                                                         %  v/v %                                                                             sec. kg/cm.sup.2                                                                        ° C                                                                        w/w %                                                                              1 24 hrs                                                                            %   kg                                __________________________________________________________________________    8   4.34.sup.(1)                                                                      7.5                                                                              0.17                                                                              30   1.2  30  0.8  0.8                                                                             14.5                                                                              99.8                                                                              0.15                              9    "  "  0.67                                                                              "    "    "   2.5  1.2                                                                             12.4                                                                              98.8                                                                              "                                  10a                                                                              4.1  7.14                                                                            0.25                                                                              40   3.0  24  8.5  1.0                                                                             12.9                                                                              99.4                                                                              "                                 11   "  "  0.65                                                                              60   "    50  4.0  --                                                                              --  --  2.0.sup.(2)                       12   "  7.0                                                                              1.2 30   "    40  18.3 1.0                                                                              7.8                                                                              99.4                                                                              0.15.sup. (3)                     13  3.47                                                                              6.0                                                                              1.25                                                                              60   "    20  3.5  --                                                                              --  --  2.0                               14  2.7.sup.(4)                                                                       5.5                                                                              20  20   "    "   35.7 --                                                                              --  --  "                                 R8  2.25                                                                              "  100 "    "    "   81.4 --                                                                              --  --  "                                 __________________________________________________________________________     NOTES:                                                                        .sup.(1) 28.5° Be at 20° C                                      .sup.(2) mold 100.sup.φ  × 170.sup.H                                .sup.(3) added carbonacious material 0.2%                                     .sup.(4) 45° Be at 20° C                                        *Flattery sands                                                          

                                      TABLE 6                                     __________________________________________________________________________                       Compression                                                    Binder         str. Kg/cm.sup.2                                                                         Retained comp. str. Kg/cm.sup.2                                                                      Broken-                                                                            Additives                                                                     Binder CO.sub.2                                                               stored                                                                        time  heated                                                                  temp. °                                                                C  down             ple    w/  w/  vl      24 S.S.I.         Ar          sands                                                                              Na.sub.2 O          No. M.R.                                                                             w %.sup.(1)                                                                       w %.sup.(2)                                                                       v % 1   HR %   200 400 600                                                                              800.sup.(3)                                                                      ArS.H..sup.(4)                                                                     Sands                                                                             gr/drop                                                                            sands               __________________________________________________________________________     9  4.34                                                                             --  7.5 0.67                                                                              1.2 12.4                                                                             98.8                                                                              --  --  --  --                                                                              8.2    F.sup.(5)                                                                       7.0  0.382                10b                                                                              4.1                                                                              --   7.14                                                                             2.5 1.5 10.9                                                                             99.4                                                                               7.8                                                                              9.1 6.0                                                                              18.7                                                                             7.1  F   7.0  0.393               12  "  --  7.0 1.2 1.0  7.8                                                                             99.4                                                                              24.7                                                                              9.0 4.2                                                                               9.5                                                                             0.8    F.sup.(6)                                                                       19.5 0.386               R9  2.25                                                                             --  6.0 100 3.4 20.3                                                                             99.7                                                                              38.1                                                                              27.3                                                                              7.2                                                                              26.8                                                                             12.6 MK.sup.(7)                                                                        0.6  0.828               R10 "  C20.sup.(8)                                                                       "   100 4.4 11.1                                                                             99.9                                                                              14.7                                                                              12.5                                                                              3.3                                                                               9.4                                                                             8.7  MK  2.5  0.690               R11 "                                                                             -- 4   "   3.5 33.1                                                                              99.8                                                                             >60 >60 10.8                                                                              >60                                                                              13.0                                                                             F    0.6 0.552                    __________________________________________________________________________     .sup.(1) % by weight on the base of alkali silicate                           .sup.(2) % binder including additives                                         .sup.(3) Ar: heated under Ar gas flow                                         .sup.(4) S.H. :simulation heating according to the real heat hysteresis       .sup.(5) F: Flattery sands(Australia)                                         .sup.(6) added carbonations material 0.2% on the base of sands                .sup.(7) MK: Mikawa sands(Japan)                                              .sup.(8) C: sucrose                                                      

                                      TABLE 7                                     __________________________________________________________________________    (Flattery Sands)                                                                                       Compression Retained compression                            Binder            strength kg/cm.sup.2                                                                      strength kg/cm.sup.2 M.sub.2 O           Ex.    Inhibitor    CO.sub.2                                                                           stored time                                                                           S.S.I.                                                                            heated temperature                                                                                 sandsee. C          No.                                                                              Alkali                                                                            M.R.                                                                             w/w %.sup.(1)                                                                      w/w %.sup.(2)                                                                      v/v %.sup.(3)                                                                      1   24 hrs                                                                            %   200  400                                                                              Ar800.sup.(4)                                                                       Ar S.H..sup.(5)                                                                      w/w                 __________________________________________________________________________                                                              %                   15 Li  3.80                                                                             --   7     0.12                                                                              1.3  6.1                                                                              98.1                                                                              1.7   1.3                                                                              0.5  <0.3   0.211                16a                                                                             Na  3.47                                                                             Am20.sup.(6)                                                                       4    1.4  2.5 12.3                                                                              99.6                                                                              10.4 11.0                                                                             12.3  3.9    0.246                16b                                                                             "   "  "    5    "    2.7 15.4                                                                              100 13.6 13.5                                                                             19.1  5.7    0.308               17 "   "  S10.sup.(7)                                                                        4    "    1.5 11.9                                                                              99.5                                                                              18.3 17.1                                                                             15.3  4.8    0.269               18 "   "  S10.sup.(8)                                                                        "    "    1.3  6.6                                                                              99.4                                                                              4.3   4.5                                                                              4.7  <1     0.246               19a                                                                              "   "  --   "    3.4  1.8 24.1                                                                              99.8                                                                              >50  21.4                                                                             26.8  <1     0.296                19b                                                                             "   "  --   3    "    2.5 19.3                                                                              99.2                                                                              46   22.2                                                                             15.6  3.8    0.222               20 "   3.66                                                                             --   4    3.5  2.9 15.6                                                                              99.6                                                                              18.3 12.1                                                                             12.6  4.3.sup.(9)                                                                          0.272               __________________________________________________________________________     .sup.(1) % by weight on the base of silicate                                  .sup.(2) % binder including admixed inhibitor on the base of sands            .sup.(3) % by volume on the base of dilute gas                                .sup.(4) Ar: under Ar gas flow                                                .sup.(5) S.H. : simulation heating according to the real heat hysteresis      of mold                                                                       .sup.(6) Am: Ammonium Silicate                                                .sup.(7) S: Colloidal silica                                                  .sup.(8) Added sucrose 10w/w % on the base of silicate                        .sup.(9) Broken-down sands: 8.3gr/drop                                   

                                      TABLE 8                                     __________________________________________________________________________                         Com-                                                                          pression                         Broken-                                                                            M.sub.2 O             Binder            str. Kg/cm.sup.2                                                                        Retained comp. str. Kg/cm.sup.2                                                                  down                                                                              Sands                   Ex.                                                                              Inhibitor    CO.sub.2                                                                           stored time                                                                         S.S.I.                                                                            heated temp. ° C                                                                          sands                                                                             w/w                     No.                                                                              M.R.                                                                             w/w %.sup.(1)                                                                      w/w %.sup.(2)                                                                      v/v %.sup.(3)                                                                      1 24 HR                                                                             %   200                                                                              400                                                                              600                                                                              Ar 800.sup.(4)                                                                     ArS.H..sup.(5)                                                                     Sands                                                                             gr/D %                  __________________________________________________________________________    21 3.47                                                                             B14.sup.(8)                                                                        4    3.5  1.7                                                                             11.3                                                                              99.8                                                                              7.4                                                                              5.3                                                                              2.3                                                                              2.4  <1   F.sup.(6)                                                                         15.0 .246               22 "  C20.sup.(9)                                                                        4    "    2.5                                                                             11.0                                                                              99.6                                                                              10.8                                                                             4.8                                                                              2.8                                                                              2.0  <1   F   18.3 "                  23 "  M20.sup.(10)                                                                       4    "    2.7                                                                             10.9                                                                              99.5                                                                              9.3                                                                              4.5                                                                              2.1                                                                              1.8  <1   F   20.8 "                  24 "  C6   6    "    2.1                                                                             12.4                                                                              99.4                                                                              6.5                                                                              9.8                                                                              2.1                                                                              2.8  1.7  MK.sup.(7)                                                                        13.3**                                                                             .418               25 "  --   6    "    1.7                                                                             20.2                                                                              99.3                                                                              10.0                                                                             5.6                                                                              4.2                                                                              4.3  3.2  MK  5.7  .443               26 "  B14  6    "    1.5                                                                              7.8                                                                              98.6                                                                              5.1                                                                              2.9                                                                              2.0                                                                              1.9  1.6  MK  14.3 .370               27 "  C3   6    1.3  2.8                                                                             11.2                                                                              99.7                                                                              9.4                                                                              4.0                                                                              2.8                                                                              4.0  5.5  *   4.3  .847               28 "  C3   6    3.5  2.5                                                                             19.7                                                                              99.8                                                                              13.0                                                                             17.0                                                                             2.8                                                                              3.0  2.2  MK  5.7  .430               29 "  --   4    3.4  1.8                                                                             24.1                                                                              99.8                                                                              >50                                                                              21.4                                                                             4.8                                                                              26.8 <1   F   7.8  .296               30 3.66                                                                             --   4    3.5  2.9                                                                             15.6                                                                              99.6                                                                              18.3                                                                             12.1                                                                             3.2                                                                              12.6 4.3  F   8.3  .272               31 4.0                                                                              B3   5.5  1.5  1.1                                                                              7.5                                                                              99.8                                                                              4.2                                                                              4.3                                                                              3.1                                                                              6.4  <1   F   11.0 .214               __________________________________________________________________________     Notes:                                                                         *used sands (1cycle)                                                         **smaller cast peice, 100m/m φ × 100m/m HGT, 2.8 Kg                 .sup.(1) % by weight on the base of the silicate                              .sup.(2) % binder including admixed inhibitor on the base of sands            .sup.(3) % by volume on the base of dilute gas                                .sup.(4) Ar: under Ar gas flow                                                .sup.(5) S.H.: simulation heating according to the real hysterisis of mol     from 1400 to 800 ° C in 1 HR                                           .sup.(6) Flattery sands, Australian                                           .sup.(7) Mikawa sands, Japanese                                               .sup.(8) B: sorbitol                                                          .sup.(9) C: sucrose                                                           .sup.(10) M: mannitol                                                    

What I claim is:
 1. In a C₂ - molding process which comprises the stepsof(a) preparing a molding composition by admixing an alkali silicatebinder to a refractory material; (b) forming the molding composition ina mold; and, (c) hardening the molding composition by applying CO₂thereto, the improvement which comprises the alkali silicate bindercomprising at least one alkali silicate selected from the groupconsisting of a sodium silicate having a molecular ratio SiO₂ /alkalimetal oxide of from 2.7 to 4.5 a potassium silicate having a molecularratio SiO₂ /alkali metal oxide of from 2.5 to 5.0 a lithium silicatehaving a molecular ratio SiO₂ /alkali metal oxide of from 2.0 to 5.5,and an alkali metal-ammonium silicate having a molecular ratio SiO₂/alkali metal oxide of from 2 to 9 and the hardening step whichcomprises gassing the molding composition with a dilute CO₂ -gasessentially consisting of CO₂ and a gas which is inert towards alkalisilicate wherein the CO₂ - concentration is not more than 20 v/v%. 2.The molding process as defined in claim 1 wherein said alkali silicatebinder has a molecular ratio of 2.7-4.5.
 3. The molding process asdefined in claim 2 wherein said alkali silicate binder has a molecularratio of 3.0-4.5.
 4. The molding process as defined in claim 1 whereinsaid alkali metal-ammonium silicate comprises a quaternary ammonium baserepresented by the formula ##STR2## wherein R₁, R₂, and R₃ eachrepresent alkyl or hydroxyalkyl containing 1-20 carbon atoms and R₄represents alkyl containing 1-4 carbon atoms.
 5. The molding process asdefined in claim 1 further comprising the step of mixing said alkalisilicate binder with at least one inhibiting agent selected from thegroup consisting of alkali-stabilized colloidal silica and a quaternaryammonium silicate.
 6. The molding process as defined in claim 5 whereinthe ratio of said alkali silicate to said colloidal silica is from about100:0.1 to about 1:2 calculated on silica.
 7. The molding process asdefined in claim 5 wherein the ratio of said alkali silicate to saidquaternary ammonium silicate is from about 100:0.1 to about 1:10calculated on silica.
 8. The molding process as defined in claim 5wherein said ammonium silicate has a molecular ratio SiO₂ /ammonium baseanhydride of about 1/2-7.
 9. The molding process as defined in claim 1further comprising the step of mixing said alkali silicate binder with0.1-30 w/w % of said alkali silicate of at least one inhibiting agentselected from the group consisting of a saccharide, a polysaccharide anda polyhydric alcohol.
 10. The molding process as defined in claim 1further comprising the step of mixing said alkali silicate binder with0.1-30 w/w % of said alkali silicate of at least one inhibiting agentselected from the group consisting of glucose, invert sugar, sucrose,sorbitol and mannitol.
 11. The molding process as defined in claim 1further comprising the step of heating said dilute CO₂ -gas to atemperature of not more than 100° C.
 12. The molding process of claim 1wherein said inert gas is dried air.
 13. The molding process as definedin claim 1, wherein the CO₂ -concentration of the dilute CO₂ -gas isfrom more than the atmospheric CO₂ concentration to not more than 20% byvolume.
 14. An alkali silicate binder composition comprising the alkalisilicate binder as defined in claim 1 and a quaternary ammonium silicatewherein the ratio of alkali silicate to ammonium silicate is from about100:0.1 to about 1:10 calculated on silica.
 15. The alkali silicatebinder composition as defined in claim 14, further comprising colloidalsilica wherein the ratio of alkali silicate to colloidal silica is fromabout 100:0.1 to about 1:2 calculated on silica.